Why are my vertices being mirrored and how do I fix it?
You have X Mirror enabled. Open the toolself with T and untick it, it's under Mesh Options. What this does is that it will try to mimic whatever you are doing on one side of your mesh on the other.



From the wiki.


  The X-mirror option of the Mesh Options panel allows you edit both “sides” of your mesh in a single action. When you transform an element (vertex, edge or face), if there is its exact X-mirrored counterpart (in local space), it will be transformed accordingly, through a symmetry along the local X axis.


This didn't affect the mouth area of your model as only half of it was modeled while you had a complete eye ring.